Method for determining the consumption of a co2 absorber in a respirator with rebreathing system

ABSTRACT

A method for determining the consumption of a CO 2  absorber ( 8 ) in a respirator with a rebreathing system, with a fresh gas mixer ( 1 ) and with a computing and control unit ( 10 ). The rebreathing system has a respiration drive ( 2 ), a volume flow sensor ( 3 ) located in the inspiratory branch, a CO 2  absorber ( 8 ) located in the expiratory branch, whose output, combined with that of the fresh gas mixer ( 1 ), is fed into the inspiratory branch, a breathing gas escape valve ( 7 ) and a breathing gas reservoir ( 9 ). The computing and control unit ( 10 ) is connected to the fresh gas mixer ( 1 ), to the respiration drive ( 2 ) and to the volume flow sensor ( 3 ) in order to receive signals and send control commands. The fresh gas volume flow V FG  discharged from the fresh gas mixer ( 1 ) and the inspiration volume flow V 1  flowing into the inspiratory branch are determined in the method in the computing and control unit ( 10 ). A value for the purified rebreathing volume flow V abs  admitted from the CO 2  absorber ( 8 ) is determined from the difference V 1 −V FG  of those values, and a rate of CO 2  absorption is determined herefrom on the basis of a preset CO 2  concentration value or from a CO 2  concentration value measured with a gas sensor in the expiratory branch and integrated over time in order to determine the quantity of CO 2  absorbed in the CO 2  absorber ( 8 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofGerman Patent Application DE 10 2006 051 571.4 filed Nov. 2, 2006, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to a method for determining theconsumption of a carbon dioxide (CO₂) absorber in a respirator(ventilator) with rebreathing system.

BACKGROUND OF THE INVENTION

Such a respirator has a fresh gas mixer, which provides the breathinggas mixture, a control unit and a rebreathing system. The rebreathingsystem has a respiration drive, e.g., a fan or a radial compressor, avolume flow sensor located in the inspiratory branch, a CO₂ absorberlocated in the expiratory branch, whose output, combined with that ofthe fresh gas mixture, is fed again into the inspiratory branch, where abreathing gas escape valve for drawing off excess breathing gas when apressure limit is reached, and a breathing gas reservoir are connected,mostly in the expiratory branch. The control unit controls the fresh gasmixer and the respiration drive and receives signals from sensors, e.g.,from the inspiratory volume flow sensor.

When the expired gas is again returned into the inspiratory branch insuch a system, the CO₂ must be removed from the expired breathing gas,which is done by means of a CO₂ absorber. Breathing lime is typicallyused as the absorber material in such a CO₂ absorber. The gas expired bythe patient flows through the breathing lime present in the CO₂absorber. The CO₂ present in the breathing gas is now absorbed by thebreathing lime and is thus removed from the gas flow. The breathing limeis depleted after a total quantity of CO₂ absorbed, which depends on thequantity of breathing lime, and no more CO₂ can be absorbed any longer.The expired CO₂ would again be introduced into the patient duringinspiration, after which correct breathing would not be guaranteed anylonger.

The consumption of breathing lime can be recognized from the change incolor of the breathing lime, which is associated with the depletion ofthe breathing lime. However, since it is undesirable, as a rule, to haveto replace the breathing lime in the CO₂ absorber or the entire CO₂absorber during an operation, it would be very useful if the staff couldbe provided with information on the state of consumption of the CO₂absorber.

A reliable and accurate calculation of the quantity of CO₂ absorberabsorbed by the CO₂ absorber is not performed in the respiratorscurrently available commercially. Thus, measurement/monitoring of thedegree of depletion of the CO₂ absorber is not yet possible in practice.An anesthesiologist can therefore use only the change in color of thebreathing lime as an indicator for the necessary replacement of thebreathing lime. Therefore, it often happens that there is a responseonly when the CO₂ content measured by a connected monitoring unit in theinspiratory air of the patient to be respirated is above preset limitvalues and corresponding alarms, derived herefrom, warn the operatingstaff.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method with whichthe cumulative quantity of absorbed CO₂ in a CO₂ absorber can bereliably determined, especially without the need for additional sensorsof instruments other than those usually already present in therespiration system.

A method is provided for determining the consumption of a CO₂ absorberin a respirator with a rebreathing system. The rebreathing system has afresh gas mixer, a computing and control unit, a respiration drive, avolume flow sensor located in the inspiratory branch, a CO₂ absorber,which is located in the expiratory branch and whose output, combinedwith that of the fresh gas mixer, is sent to the inspiratory branch, abreathing gas escape valve and a breathing gas reservoir. The computingand control unit is connected to the fresh gas mixer, the respirationdrive and the volume flow sensor to receive signals and to send controlcommands. The fresh gas volumes discharged from the fresh gas mixer andthe inspiration volume flow flowing into the inspiratory branch aredetermined according to the method in the computing and control unit. Avalue for the purified rebreathing volume flow added from the CO₂absorber is determined from the difference of those values, and a rateof CO₂ absorption is determined and integrated over time from this onthe basis of a preset CO₂ concentration value or a CO₂ concentrationvalue measured with a gas sensor in the expiratory branch in order todetermine the quantity of the CO₂ absorbed in the CO₂ absorber.

The present invention is based on determining the balance of the volumeflows in the respiration system as accurately as possible in order todetermine the quantity of CO₂-containing expired gas, which has flownthrough the CO₂ absorber, as accurately as possible, in order todetermine from this the quantity of absorbed CO₂ by means of a value ofthe CO₂ concentration in the expired breathing gas.

To determine the balance of the volume flows through the CO₂ absorber,the fresh gas volume flow and the inspiration volume flow flowing in theinspiratory branch are determined. Since the inspiration volume flow iscomposed of the fresh gas volume flow and the purified breathing gasadded from the CO₂ absorber volume flow, a value can be determined onthe basis of the difference between the inspiratory volume flow and thefresh gas volume flow for the purified rebreathing volume flow addedfrom the CO₂ absorber, the added volume flow being determined accordingto the general aspect of the present invention by integrating thepositive components of V_(i)−V_(FG), i.e., negative refluxes to theabsorber remain ignored in the general embodiment.

The quantity of CO₂ absorbed in the CO₂ absorber can be calculated bymeans of the volume flow discharged from the CO₂ absorber and the CO₂concentration in front (upstream) of the absorber. The absorbed CO₂concentration of the absorber is approximately equal to the product ofthe volume flow from the absorber and the CO₂ concentration in front ofthe absorber. However, a more accurate calculation is preferably carriedout, in which the volume flow reduction in the CO₂ absorber due to theabsorption of CO₂ is taken into account and the volume flow V^(in)_(abs) entering the CO₂ absorber is calculated; this can be carried outby means of the equation:

${\overset{.}{V}}_{abs}^{in} = {{\overset{.}{V}}_{abs}\left( \frac{100}{100 - {{{Vol}.\mspace{11mu} \%}\mspace{11mu} \left( {CO}_{2} \right)}} \right)}$

At a CO₂ content of 5 vol. %, this means that the volume flow in frontof the CO₂ absorber is 100/95=1.053 times greater than the observedvolume flow behind (downstream of) the CO₂ absorber. The product of thisvolume flow V^(in) _(abs) entering the CO₂ absorber and the CO₂concentration in front of the absorber yields the quantity of CO₂absorbed in the absorber per unit of time.

The above-described procedure for determining the quantity of absorbedCO₂ basically somewhat overestimates the actual quantity because it isnot taken into account that CO₂-free gas can flow through the CO₂absorber against the direction of the expired breathing gas duringcertain phases, so that a CO₂-free volume can form in front of the CO₂absorber, and this volume will first flow through the absorber andcontribute to the volume flow from the CO₂ absorber only during the nextexpiration phase, without, however, depositing CO₂ in the absorber. Thishappens especially when operating with a continuous fresh gas volumeflow. No inspiratory volume flow is flowing at times outside theinspiration phase, so that the fresh gas flowing in continuously in theopposite direction flows through the CO₂ absorber and further throughthe line up to an anesthetic gas escape valve. This entire volume infront of the CO₂ absorber up to the anesthetic gas escape valve canconsequently be filled with CO₂-free gas, which is pushed through theCO₂ absorber first during the next inspiration cycle. This maximumCO₂-free volume will hereinafter also be called buffer volume. It mustbe determined and stored in advance for each rebreathing systemdepending on the design, dimensions and line connections of thisrebreathing system. During phases during which the volume flow V_(FG)from the fresh gas mixer is greater than the inspiratory flow V_(i),fresh gas flowing off in the direction of the CO₂ absorber is preferablybalanced as a CO₂-free volume in front of the CO₂ absorber byintegrating the volume flow (V_(FG)−V_(i)) flowing through the CO₂absorber up to the preset maximum buffer volume and storing it as aCO₂-free volume value, and the CO₂-free volume flow is subtracted fromthe integrated volume flow through the absorber during the nextinspiration phase, when the inspiratory volume flow V_(i) is greaterthan the fresh gas volume flow V_(FG).

The control unit is preferably set up in the method to receive and storethe maximum quantity of absorbed CO₂ which the CO₂ absorber can absorband/or to initiate the sending of an audio signal upon input by anoperator, as soon as the integrated quantity of absorbed CO₂ exceeds thestored maximum.

The present invention will be described below on the basis of anexemplary embodiment in connection with the drawings. The variousfeatures of novelty which characterize the invention are pointed outwith particularity in the claims annexed to and forming a part of thisdisclosure. For a better understanding of the invention, its operatingadvantages and specific objects attained by its uses, reference is madeto the accompanying drawings and descriptive matter in which a preferredembodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view showing the design of a rebreathing systemfor use with the present invention; and

FIG. 2 is a graph with the volume flows and the volume passing throughthe CO₂ absorber as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in particular, FIG. 1 shows as an example theschematic design of a rebreathing system suitable for the application ofthe present invention. The arrows indicate the direction of a gas flowto and from an element. The valves guaranteeing the direction of gasflow are not shown here because these are irrelevant for the principleof determining the CO₂ absorber consumption being described here, onlythe directions of the particular gas flows being relevant. Broken lineswithout an indicated direction indicate an electric connection or a datacommunications section, along which information is transported fromelements of the rebreathing system to a central computing and controlunit 10 or also the other way around.

A gas mixture (usually composed of the individual gases O₂, air, N₂O,volatile anesthetic gases) are sent in the fresh gas mixer to therebreathing system corresponding to the setting made by the user or theoperator. This gas flow is usually called “fresh gas.” The volume of thefresh gas volume flow is reported back to the computing and control unit10. The fresh gas mixer 1 may be a mechanical mixer, which is equippedwith an electronic gas volume flow measurement, or also an electronicmixer, which receives the metering data from the computing and controlunit 10.

On the way to the respiration drive, the fresh gas is mixed with thevolume expired during the expiration, which originates from the patient4 and is freed of CO₂ in the CO₂ absorber, and is delivered to thepatient 4 through the (inspiratory) volume flow measuring unit 3 in therespiration drive in a volume- and/or pressure-controlled manner. The(inspiratory) volume flow measuring unit 3 sends the volume flowinformation V_(i) measured in the method to the computing and controlunit 10.

After the inspiration phase, the patient 4 can again release an expiredexpiration volume to the rebreathing system through the (expiratory)volume flow measuring unit 5. The (expiratory) volume flow measuringunit 5 sends the volume flow information measured in the process to thecomputing and control unit 10.

The expiratory volume flow expired by the patient is split intodifferent branches within the rebreathing system. One part is admittedinto the breathing gas reservoir 9 (this may be identical to parts ofthe respiration drive in other systems), from which it is then availableat the beginning of the next inspiration phase for the respiration drive2, flowing through the CO₂ absorber 8.

Another part of the expired breathing gas is fed to an anesthetic gasescape through the anesthetic gas escape valve 7. The anesthetic gasescape valve 7 opens only beginning from a defined internal pressure inthe system, so that the breathing gas reservoir 9 must first be filledcompletely before anesthetic gas is lost from the system.

The gas concentrations (O₂, CO₂, N₂O, volatile anesthetic gases) aremeasured during the breathing (respiration) at the point at which theinspiratory and expiratory connection of the rebreathing system isconnected to the patient 4 (the so-called “Y-piece”). This may becarried out by a so-called suctioning (sidestream) gas measurement oralso a directly measuring gas measurement integrated in or connected tothe Y-piece. This is shown in FIG. 1 by the suctioning gas measurement,which feeds the gas flow drawn off to the rebreathing system.

The proportional breathing lime consumption is determined, on the onehand, by the user setting the maximum quantity of CO₂ to be absorbed perabsorber filling in a configuration menu on the computing and controlunit 10 and, on the other hand, by a volume flow balancing, by which theCO₂ volume absorbed in the CO₂ absorber 8 is determined.

The user or the operator must likewise communicate for this to thesystem (e.g., by manual acknowledgment on the computing and controlsystem 10) when the breathing lime in the CO₂ absorber 8 (or the CO₂absorber 8 as a whole) was changed. If only a certain CO₂ absorber 8with, e.g., a defined, preset breathing lime can be used or is releasedfor the particular rebreathing system, the limit of the maximum volumeof CO₂ to be absorbed by this CO₂ absorber may likewise be preset by themanufacturer of the entire system already at the time of the sale of thedevice (e.g., in the computing and control unit 10).

At another level of expansion, CO₂ absorbers 8 may be provided, e.g.,with radio frequency identification (RFID) tags, so that an electronicunit integrated in the control and computing unit 10 (or connectedthereto externally) automatically recognizes the replacement of the CO₂absorber 8; the manual acknowledgement of the replacement of thebreathing lime, which is otherwise necessary on the part of the user,would thus become unnecessary.

The calculation of the CO₂ volume absorbed by the CO₂ absorber 8 is thenrestarted at a value of “0” after the breathing lime replacementannounced by the computing and control unit 10 and continued/integrateduntil the next absorber replacement (also beyond the switching on andoff of the device).

The calculation of the total volume of CO₂ absorbed by the CO₂ absorber8 is carried out primarily on the basis of the inspiratory volume flowV₁, which is measured by means of the inspiratory volume flow measuringunit 3 or is derived from the setting of the respiration drive 2, whichsetting is known to the control unit, and the fresh gas volume flowV_(FG) (through the fresh gas mixer 1, which is known to the computingand control unit 10). Any volume that is fed to the patient during theinspiration phase must consist of the addition of the volume taken fromthe fresh gas mixer 1 and the CO₂ absorber 8.

The quantity of (patient) gas previously enriched with CO₂ that hasflown through the CO₂ absorber 8 is thus known, in principle:V_(abs)=V₁−V_(FG). The percentage of CO₂ in the air expired by thepatient can be determined by means of the gas measurement 6, so that thepercentage of the CO₂ volume reacted in the CO₂ absorber can bedetermined by means of this CO₂ concentration and the total volume(determined according to the above) that has flown through the CO₂absorber.

As an alternative (resulting in higher inaccuracy), the gas measurementmay also be eliminated because the CO₂ value of respirated patients isonly in a relatively narrow possible range during average respiration.The value of the CO₂ concentration, which is to be taken into account,can thus be preset as a fixed value or it may also be configured by theuser.

The inspiratory volume flow fed to the patient during inspiration canalso be replaced or supplemented with information from the respirationdrive 2 instead of information from an inspiratory volume flowmeasurement 3.

If the fresh gas volume flow is fed by the fresh gas mixer 1 to therebreathing system continuously, it should also be taken into account(also as a function of the position of the manual respiration bag 9present in the rebreathing system), to increase the accuracy of thealgorithm, that a volume flow is flowing through the CO₂ absorber 8 inthe direction of the anesthetic gas escape valve 7 during the patient's“non-inspiration phase.”

The volume between the CO₂ absorber 8 and the anesthetic gas escapevalve 7 is thus replaced by CO₂-free air. The CO₂ absorber 8 does nothave to free this component of the CO₂-free volume of CO₂ at thebeginning of the next inspiration phase, so that this volume percentageis subtracted from the reacted CO₂ volume (determined according to theabove without this correction) in a preferred embodiment.

This shows clearly that the design of the particular rebreathing systembeing considered must be taken into account when forming the model forthe determination of the CO₂ volume absorbed by the CO₂ absorber,because the total volume between the CO₂ absorber 8 and the anestheticgas escape valve 7 depends on the line volumes of the particularrebreathing system and must be determined and stored in advance, becauseit sets the maximum buffer volume with CO₂-free gas in front of the CO₂absorber 8.

The calculation shall be explained in more detail in FIG. 2 on the basisof the rebreathing system outlined schematically in FIG. 1. The volumeflow to and from the manual respiration bag is ignored here, becausethis can be minimized by other measures (see DE 100 41 007 C1); thepercentage of CO₂ expired by the patient is likewise assumed to beconstant in this illustration.

The volume flows that are relevant for this calculation in the systemare plotted on the upper curve. The volume flows flowing to the patientare assumed to be constant here for illustration (e.g., duringrespiration with constant volume), and they are represented by theinspiration volume flow V₁ 21. The expiration volume flow is designatedby 24. The fresh gas volume flow V_(FG) supplied by the fresh gas mixer1 is designated by 23.

The calculated volume ratios for the CO₂ absorber 8 are plotted on themiddle curve. If a volume flow containing CO₂ is delivered through theCO₂ absorber, a CO₂ volume is integrated (positive range of the CO₂volume curve 26, illustrated as an area drawn by broken lines under theCO₂ volume curve 26). The negative range of curve 26 designates thecurrent CO₂-free volume in the buffer volume between the CO₂ absorber 8and the intersection 11.

The phases of the respiration cycles just described are shown on thelower curve. The inspiration volume flows 21 are equal and of equalduration in all three inspiration phases (31, 33, 35) shown. Thisresults in a total inspiration volume which is equal for each breathingstroke.

Phase 31 (Inspiration):

The fresh gas volume flow 23 is relatively high, and only a certainvolume flow 22 must therefore be delivered through the CO₂ absorber 8.

The starting condition assumed in this calculation is that the volumebetween the absorber 8 and the intersection 11 is completely free of CO₂before this first breathing stroke. This volume is called the maximumbuffer volume 25 here.

If volume is now delivered through the CO₂ absorber 8 in the directionof the patient, CO₂-free gas will first flow through the CO₂ absorber 8(corresponding to the size of the maximum buffer volume), represented bya rise of curve 26. If the volume flowing through the CO₂ absorber 8 isgreater than the CO₂-free volume present in the buffer volume,CO₂-containing gas will be delivered from the (preceding) expirationphase through the CO₂ absorber 8 and absorbed in the CO₂ absorber 8. Asa result, the remaining absorption capacity of the CO₂ absorber 8 iscorrespondingly reduced. The CO₂ content in the expiration air of thepreceding expiration phase is decisive here. The fact that the volume ofCO₂-containing air decreases on its way through the CO₂ absorber 8because the CO₂ is extracted may likewise be taken into account in thecalculation. This means that the volume of CO₂-containing air enteringthe CO₂ absorber 8 must be larger than the volume of CO₂-free air thatis discharged and the percentage of absorbed CO₂ volume is thuscorrespondingly increased.

Phase 32 (Expiration):

After the end of the flow phase during the inspiration, the continuouslyflowing gas volume flow 23 is not delivered to the patient any longer,i.e., it must be sent through the absorber 8 in the direction of theanesthetic gas escape valve 7. The buffer volume is flushed withCO₂-free air now. The CO₂-free volume, which is thus present in thebuffer volume until the inspiration phase begins the next time, dependson the fresh gas volume flow 23 set and the time available until thenext inspiration phase. However, the maximum volume is limited here bythe space present in the respiration system due to the design betweenthe CO₂ absorber 8 and the intersection 11, represented as a maximumbuffer volume 25 here. The maximum buffer volume of CO₂-containing airis purified during phase 32 (due to the relatively high setting of thefresh gas volume flow 23 and the long time of this phase).

Phase 33 (Inspiration):

This phase is identical to phase 31 here.

Phase 34 (Expiration):

The fresh gas volume flow 23 is reduced at the beginning of this phase.The buffer volume is thus freed of CO₂ to a lesser extent only comparedto what happened in phase 32; the maximum buffer volume is not utilizedin this case.

Phase 35 (Inspiration):

A certain percentage of CO₂-free gas is delivered through the CO₂absorber 8 here as well, but less than in the preceding phase 33,because the CO₂ absorber-free volume buffered was smaller. Therefore,more CO₂-free gas is delivered through the CO₂ absorber 8. In addition,because the fresh gas volume flow 23 was reduced, a larger volume mustbe delivered through the CO₂ absorber 8; this can be recognized from asharper rise of curve 26 compared to phase 33. Both lead to thecircumstance that a considerably larger volume of CO₂ must be absorbedin the CO₂ absorber 8 in phase 35 than in phases 31 and 33.

The CO₂ volumes thus determined for individual breathing strokes canthen be integrated and thus they represent the quantity of CO₂ absorbedby the CO₂ absorber 8.

If the maximum quantity of CO₂ absorbed, which was set by the user orthe operator on the computing and control unit 10, has been reached orexceeded, the entire device can inform the person setting up the entiredevice visually and/or audibly that the maximum CO₂ absorption capacityof the connected CO₂ absorber 8 is exceeded and the latter (or thebreathing lime therein) must therefore be replaced. Otherwise, e.g., thestill remaining residual capacity of the breathing lime may be displayedto the user.

In addition, the user or the operator may possibly also set on thecomputing and control unit 10 by how much sooner than the set maximumabsorption capacity of the breathing lime the reports shall occur duringthe setting up (and/or during the later operation of the entire device).The user can then determine whether warning should be given rather early(=meaning maximum safety for the patient) or rather late (=meaningmaximum utilization of the breathing lime).

The residual capacity of the breathing lime may be displayed, e.g., as apercentage value (expressing the ratio of the CO₂ volume alreadyabsorbed to the maximum limit set by the user), as a still remainingvalue of CO₂ volume that can still be absorbed (indicating the quantity,e.g., in liters), or also as a still remaining residual time of the CO₂absorber. If the residual time is indicated, the underlying algorithm asthe basis of the time indication can calculate, e.g., the time withinwhich (during ongoing respiration) a certain volume of CO₂ was absorbedby the CO₂ absorber 8 in the past of the entire device (based on theassumption that the user/the entire device will continue to behavecomparably concerning the CO₂ absorption characteristic in the CO₂absorber as in the past in case of similar settings).

If necessary, the user or the operator may also be provided with thewarnings described above during the ongoing operation of the device inthe form of, e.g., visual/audible alarm reports.

Furthermore, the date of the first use of this particular CO₂ absorber 8can be written, e.g., on the RFID tag in the case of a CO₂ absorber 8equipped with, e.g., an RFID tag. If a CO₂ absorber 8, which already hasa set date on the RFID tag, is then connected to a rebreathing system,the computing and control unit can visually and/or audibly warn the userthat a CO₂ absorber that had already been used was connected to theparticular entire device.

The information that the CO₂ absorber 8 has already been used can bewritten, e.g., on an RFID tag with indication of the date of the firstuse or also as a simple flag or also in any other form (and it can thenbe read by the computing and control unit 10).

If the date of first use is written on, e.g., the RFID tag of the CO₂absorber 8, the date thus stated can be used, furthermore, e.g., toinform the person performing the set-up, e.g., during the setting up ofthe entire device, visually and/or audibly that, e.g., a maximum usetime of the connected CO₂ absorber 8, which is to be set, e.g., by theuser on the computing and control unit 10, and after which the CO₂absorber 8 shall be replaced (e.g., for hygienic reasons) at the latest,has been exceeded. If necessary, this warning may also be provided forthe user during the ongoing operation of the device in the form of,e.g., visual/audible alarm reports.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A method for determining the consumption of a CO₂ absorber in arespirator with a rebreathing system, the method comprising the stepsof: providing the rebreathing system with a fresh gas mixer, with acomputing and control unit, a respiration drive, a volume flow sensorlocated in an inspiratory branch, a CO₂ absorber located in theexpiratory branch, the CO₂ absorber providing an output that is combinedwith an output of said fresh gas mixer and is sent to said inspiratorybranch, a breathing gas escape valve and a breathing gas reservoir;connecting said computing and control unit to said fresh gas mixer, saidrespiration drive and said volume flow sensor to receive signals and tosend control commands; determining fresh gas volume (V_(FG)) dischargedfrom said fresh gas mixer and inspiration volume flow (V₁) flowing intothe inspiratory branch in said computing and control unit; determining avalue for the purified rebreathing volume flow (V_(abs)) added from saidCO₂ absorber from a difference of the values of the inspiration volumeflow and the fresh gas volume (V₁−V_(FG)); determining from the valuefor a purified rebreathing volume flow (V_(abs)) a rate of CO₂absorption; integrating the determined rate of CO₂ absorption over timeon the basis of a preset CO₂ concentration value or a CO₂ concentrationvalue measured with a gas sensor in the expiratory branch in order todetermine the quantity of the CO₂ absorbed in said CO₂ absorber.
 2. Amethod in accordance with claim 1, further comprising performing acorrection, which takes into account the fact that said volume flow(V_(abs)) discharged from said CO₂ absorber is smaller than the volumeflow entering said CO₂ absorber because of the CO₂ absorption in saidCO₂ absorber, during the calculation of the rate of CO₂ absorption onthe basis of said volume flow (V_(abs)) discharged from said CO₂absorber and the CO₂ concentration.
 3. A method in accordance with claim1, further comprising: balancing the fresh gas flowing off in thedirection of said CO₂ absorber as CO₂-free volume upstream of said CO₂absorber, during continuous inflow of fresh gas from said fresh gasmixer during phases during which said volume flow from said fresh gasmixer (V_(FG)) is greater than said inspiratory volume flow (V₁), byintegrating the volume flow flowing through said CO₂ absorber up to apreset maximum buffer volume and storing it as a CO₂-free volume valueand by subtracting the value of the CO₂-free volume from the integratedvolume flow through said CO₂ absorber in the next inspiration phase,when said inspiratory volume flow (V₁) is greater than said fresh gasvolume flow (V_(FG)).
 4. A method in accordance with claim 1, whereinsaid computing and control unit is set up to receive and store themaximum quantity of absorbed CO₂ which said CO₂ absorber can absorb uponan input by an operator, and wherein said computing and control unitinitiates the ending of a visual and/or audible signal as soon as theintegrated quantity of absorbed CO₂ exceeds the stored maximum value. 5.A method in accordance with claim 4, wherein said control unit initiatesa visual display of the still remaining absorption capacity of said CO₂absorber on the basis of the integrated quantity of absorbed CO₂ and themaximum quantity of absorbed CO₂.
 6. A method in accordance with claim5, wherein the residual capacity is displayed as a percentage of theintegrated quantity of absorbed CO₂ compared to the maximum quantity ofabsorbed CO₂, as a remaining value of the CO₂ absorption quantity or asa still remaining residual operating time of said CO₂ absorber until themaximum quantity of CO₂ is reached in linear extrapolation of the pastrate of CO₂ absorption.
 7. A method in accordance with claim 1, whereinsaid computing and control unit is connected to an RFID writing andreading device and said CO₂ absorber is equipped with an RFID tag,wherein said computing and control unit automatically recognizes thereplacement of said CO₂ absorber on the basis of the information readfrom a RFID transponder.
 8. A method in accordance with claim 7, whereinsaid computing and control unit stores the current date as the date offirst use in a preset storage location when no valid date is still beingstored there, or in which said computing and control unit stores Booleaninformation indicating the use as a flag in the RFID transponder at thepreset storage location when said CO₂ absorber is put into use.
 9. Amethod in accordance with claim 8, wherein said computing and controlunit reads the date of first use or the other flag, which was previouslystored for this purpose and indicates the fact of use, as a Booleaninformation from the RFID transponder of said CO₂ absorber and providesa visual and/or audible display, which alerts an operator if said CO₂absorber is a, previously used CO₂ absorber.
 10. A method in accordancewith claim 1, wherein said computing and control unit polls an operatorif replacement of said CO₂ absorber has taken place and, if yes, toreset to zero the integrated quantity of CO₂ for such said CO₂ absorber.11. A method in accordance with claim 7, wherein said computing andcontrol unit automatically resets to zero the integrated quantity ofabsorbed CO₂ when replacement of said CO₂ absorber is detected, and ifan already used CO₂ absorber is detected, said computing and controlunit prompts the operator to enter the quantity of CO₂ already absorbedin said used CO₂ absorber for said CO₂ absorber as a starting value forthe further integration of the absorbed quantity of CO₂.
 12. Arespiration method comprising the steps of: providing a rebreathingsystem with a fresh gas mixer, a computing and control unit, arespiration drive, a volume flow sensor located in an inspiratorybranch, a CO₂ absorber located in the expiratory branch, the CO₂absorber providing an output that is combined with an output of saidfresh gas mixer and is sent to said inspiratory branch, a breathing gasescape valve and a breathing gas reservoir; connecting said computingand control unit to said fresh gas mixer, said respiration drive andsaid volume flow sensor to receive signals and to send control commands;determining a fresh gas volume discharged from said fresh gas mixer;determining an inspiration volume flow flowing into the inspiratorybranch in said computing and control unit; determining a value for apurified rebreathing volume flow added to the inspiration volume flowfrom said CO₂ absorber based on a difference of the values of theinspiration volume flow and the fresh gas volume; and determining fromthe value for the purified rebreathing volume flow a rate of CO₂absorption.
 13. A method in accordance with claim 12, furthercomprising: integrating the determined rate of CO₂ absorption over timeon the basis of a preset CO₂ concentration value or a CO₂ concentrationvalue measured with a gas sensor in the expiratory branch in order todetermine the quantity of the CO₂ absorbed in said CO₂ absorber.
 14. Amethod in accordance with claim 12, further comprising performing acorrection including taking into account the volume flow discharged fromsaid CO₂ absorber that is smaller than the volume flow entering the CO₂absorber in calculating the rate of CO₂ absorption.
 15. A method inaccordance with claim 12, further comprising: detecting a fresh gas flowphase in which said volume flow from said fresh gas mixer is greaterthan said inspiratory volume flow; integrating the volume flow flowingthrough said CO₂ absorber, up to a preset maximum buffer volume, duringthe fresh gas flow phase and storing the integrated the volume flowflowing through said CO₂ absorber up to a preset maximum buffer volumeas a CO₂-free volume value; and subtracting the CO₂-free volume valuefrom the integrated volume flow through said CO₂ absorber in the nextinspiration phase in which said inspiratory volume flow is greater thansaid fresh gas volume flow.
 16. A method in accordance with claim 12,wherein said computing and control unit is set up to receive and storethe maximum quantity of absorbed CO₂ which said CO₂ absorber can absorbupon an input by an operator.
 17. A method in accordance with claim 16,wherein said computing and control unit initiates a visual display ofthe still remaining absorption capacity of said CO₂ absorber on thebasis of the integrated quantity of absorbed CO₂ and the maximumquantity of absorbed CO₂.
 18. A method in accordance with claim 12,wherein said computing and control unit is connected to an RFID writingand reading device and said CO₂ absorber is equipped with an RFID tag,wherein said computing and control unit automatically recognizes thereplacement of said CO₂ absorber on the basis of the information readfrom the RFID transponder.
 19. A respiration system comprising: arebreathing system with a fresh gas mixer, a computing and control unit,a respiration drive, a volume flow sensor located in an inspiratorybranch, a CO₂ absorber located in the expiratory branch, the CO₂absorber providing an output that is combined with an output of saidfresh gas mixer and is sent to said inspiratory branch, a breathing gasescape valve and a breathing gas reservoir, said computing and controlunit being connected to said fresh gas mixer, to said respiration driveand to said volume flow sensor to receive signals and to send controlcommands; wherein said computing and control unit determines a fresh gasvolume discharged from said fresh gas mixer, determines an inspirationvolume flow flowing into the inspiratory branch in said computing andcontrol unit and determines a value for a purified rebreathing volumeflow added to the inspiration volume flow from said CO₂ absorber basedon a difference of the values of the inspiration volume flow and thefresh gas volume and determines from the value for the purifiedrebreathing volume flow a rate of CO₂ absorption; and wherein saidcomputing and control unit integrates the determined rate of CO₂absorption over time on the basis of a preset CO₂ concentration value ora CO₂ concentration value measured with a gas sensor in the expiratorybranch in order to determine the quantity of the CO₂ absorbed in saidCO₂ absorber.
 20. A system in accordance with claim 19, furthercomprising: a radio frequency identification (RFID) reading device,wherein said computing and control unit is connected to said RFIDreading device; and an RFID transponder, said RFID transponder beingassociated with said CO₂ absorber, wherein said computing and controlunit automatically recognizes the replacement of said CO₂ absorber onthe basis of the information read from the RFID transponder.